Question

During the equilibrium experiment at room temperature, you prepare the standard solution of FeSCN²⁺ by taking 9.00 mL of 0.1990 M Fe(NO₃)₃ from the dispenser into a 20 x 150 mm test tube. You pipet 1.00 mL of 0.001904 M KSCN into the same test tube and stir. The absorbance of this solution registers as 0.786 au on the spectrometer. Determine [FeSCN²⁺]_eq with units M and to 6 decimal places.

Answer 1

Chemical reaction involved:

Fe³⁺ + SCN^- Fe(SCN)²⁺

Number of millimoles of Fe³⁺in the solution = Molarity of solution Volume added (ml) = 0.1990 9 = 1.791

Number of millimoles of SCN^- in the solution = Molarity of solution Volume added (ml) = 0.001904 1 = 0.001904

From the stochiometric equation we can see that SCN^- is the limiting reagent.

Equilibrium millimoles of Fe(SCN)²⁺ = Initial millimoles of SCN^- = 0.001904

Molarity of Fe(SCN)²⁺ formed at eqilibrium = millimoles / total volume of solution (ml) = 0.001904 / 10 = 0.000190 M

Number